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Posted by: algorimancer Jun 20 2011, 03:02 PM

I got to wondering about this, considered making my own graphic, then googled and came up with the following, from Burton Mackenzie's blog:

http://1.bp.blogspot.com/_tlXkPX0vMqk/SaTWYkrU6pI/AAAAAAAAAM0/Za2Ngvg5q_s/s400/sun_size_diagram.png

So apparently the Sun appears about 3 times as large in Mercury's sky as it does on Earth. I was expecting something much more dramatic.

Posted by: ilbasso Jun 20 2011, 03:18 PM

That is interesting indeed. It reminds us how far away even Mercury is from the Sun. So many graphics of the solar system make it look like Mercury is only a couple solar radii away!

I had a similar question a couple of months ago, wondering how far you would have to be from the Moon in order for it to appear the same size as Earth from your vantage point. It was a surprisingly small distance, only about 30,000 miles, if I remember correctly. Both bodies would be about 4 degrees across as seen in your sky.

Posted by: Decepticon Jun 20 2011, 09:14 PM

I have a question thats kinda related.

How big does Jupiter look from Io's Surface?
Europa also!

Posted by: Phil Stooke Jun 20 2011, 09:23 PM

It's quite simple geometry. For the same object (like the moon), double the distance = half the size, half the distance = double the size and so on, and for different objects at the same distance, twice the size = twice the apparent diameter and so on.

So... for the sun, Mercury's a third of the distance that Earth is from the sun, so three times bigger is what you'd expect.

Jupiter from Io? Jupiter is about 40 times the diameter of the Moon (very roughly) so from the Moon's distance it would look 40 times bigger than the Moon. Then multiply by the appropriate fraction for the ratio of distances - you can go from there yourself, I think.

Phil

Posted by: Gsnorgathon Jun 20 2011, 09:53 PM

Bravo, Phil! That's much better than my answer - I was just going to say 2arctan(radius/distance), but your version's a lot easier to work out in one's head.

Apparent diameter in degrees (Moon from Earth = .5):
Jupiter from Io 18.6
Jupiter from Europa 11.8
Jupiter from Ganymede 7.4
Jupiter from Callisto 4.2

Posted by: Mr Valiant Jun 26 2011, 01:00 AM

Hey! Jupiter is so big. 4.2 degrees from Callisto, and considering Callisto is pretty much outside
the Jupiter Van Allen belts, it may be the first place humans set foot in the Jupiter system.
Will be an awesome sight.

Posted by: Decepticon Jun 26 2011, 04:29 AM

Interesting, Most space art of Jupiter from Io or Europa look incorrect.

Posted by: Greg Hullender Jun 26 2011, 05:04 PM

Well, most Earth art makes the moon way, way too big, so I'm not sure what space art is supposed to do. I do get a kick out of how when they show Saturn from any of its moons, artists frequently tilt the planet so the rings can be seen.

Another point on sizes--forgive me if this seems too obvious--but the area is propotional to the square of the diameter. So the sun is not 3x bigger at Mercury--it's 9x bigger.

--Greg

Posted by: JohnVV Jun 27 2011, 08:47 AM

a quick look
from Earth
[attachment=24650:sun.earth.jpg]
from Mercury
[attachment=24649:sun.mer.jpg]
earth
[attachment=24651:earth.jpg]
Mercury
[attachment=24652:mer.jpg]

Posted by: Bjorn Jonsson Jun 28 2011, 09:41 AM

This doesn't have to be the case - the field of view varies. There is no single correct size of Jupiter as seen from the Galileans.

Posted by: djellison Jun 28 2011, 03:29 PM

Quite. Neither of these views is 'incorrect'

Posted by: Phil Stooke Jun 28 2011, 04:45 PM

"So the sun is not 3x bigger at Mercury--it's 9x bigger."

Let's not confuse diameter and area! Both are correct.

Phil

Posted by: ilbasso Jun 28 2011, 08:41 PM

To be precise, the Sun is the same size. It just LOOKS bigger.

Posted by: Fran Ontanaya Jun 29 2011, 02:41 AM

I think it would still have a smaller apparent area from Earth after compensating for the distance, due to Lorenz contraction as the Sun moves across the line of view.

Posted by: tedstryk Jun 29 2011, 09:58 PM

Huh? So are you saying that as I move my hand away from my face my hand doesn't shrink, it just looks smaller? My whole cosmology has just been rocked!

Posted by: djellison Jun 29 2011, 10:22 PM

Every time a pointless debate of semantics begins, a baby seal pup gets clubbed to death

True story.

Posted by: ElkGroveDan Jun 30 2011, 01:51 PM

That reminds me Doug, your pelts from last month are ready. The guy says he's OK with coats and hats, but there is no way he can stitch them into undershorts. Sorry.

Posted by: ilbasso Jun 30 2011, 01:56 PM

I need to be better about using smiley faces.

Posted by: hendric May 16 2018, 08:26 PM

Are there any good videos of the sun throughout a Mercurian day? I found this video, but it doesn't show how the sun's location varies, or what a specific observer on a specific spot would see. I assume there are solar system simulators that would allow one to stand on spots on Mercury and watch the sun progress?

https://www.youtube.com/watch?v=gubNWJ5RlP4

I assume at one of the sub-solar points you would see the Sun approach overhead, slow down, get larger, pause for a bit, then speed up and get smaller. But what would a person not at one of those points see? If you were 90* away, the sun would be largest at sunrise, shrink, then get larger again at sunset?

Thanks everyone!

Posted by: Sean May 16 2018, 09:01 PM

Space Engine will show you.

http://spaceengine.org/

Posted by: Gustavo B C May 17 2018, 06:48 AM

http://stellarium.org/ as well is pretty good. F6 to change location, switch planet to Mercury, pick somewhere on the map and speed up time as you wish. You can zoom into the Sun and watch it change size as time progresses, change your location 90° to the east/west and do the same again. Spoiler alert: your assumption is correct

Curiously, just over a year ago I made a gif/short video using Stellarium (https://gfycat.com/FairBrilliantIndianjackal) of what a solar day on Mercury would look like, but the change in apparent size of the Sun isn't really noticeable - it was more aimed at visualising the Sun's movement through the sky. The labels are all in Portuguese, L is east and O is west.

Posted by: hendric May 17 2018, 09:52 PM

Thanks for tips, those both look like great programs!

Posted by: djellison May 18 2018, 12:30 AM

https://eyes.nasa.gov could do it as well. If you use the shoulder camera on Mercury then lock the camera to the sun you could watch it all day.